Tolérances CNC expliquées : guide pratique GD&T pour 2026
Que signifie réellement ±0,005 mm ? Guide pratique d'ingénieur sur tolérances CNC, cumul, bases GD&T, comment spécifier et inspecter.
Spécifier des tolérances est le plus grand levier sur le coût d'une pièce CNC. Trop lâche, la pièce ne fonctionne pas. Trop serré partout, le prix triple sans bénéfice. Ce guide explique ce que signifient les tolérances en termes d'usinage réel.
What a tolerance really is
A tolerance is the allowable variation in a dimension. If a drawing says “25.00 ±0.05 mm”, the part is acceptable when the measured dimension falls between 24.95 and 25.05 mm.
Three things determine the achievable tolerance on any given feature:
- Machine capability. A 20-year-old manual mill holds ±0.1 mm at best. A modern Mazak 5-axis cell holds ±0.005 mm comfortably. The machine sets the floor.
- Material behaviour. Aluminium machines stably; titanium springs back; thin-walled stainless deflects under cutter pressure. Material doubles or halves the achievable tolerance.
- Feature geometry. Tolerances on a 5 mm feature near the chuck are easy. Tolerances on a 200 mm-long thin wall are hard. Geometry can make the same nominal dimension 10× harder to hold.
What CNC can actually hold
| Feature type | Standard CNC | Precision CNC | High-end (with care) |
|---|---|---|---|
| External dimensions | ±0.10 mm | ±0.025 mm | ±0.005 mm |
| Hole diameter | ±0.05 mm | ±0.013 mm | ±0.005 mm |
| Threaded holes (pitch) | 6H class fit | 5H class fit | Custom |
| Surface finish (Ra) | 3.2 µm | 0.8 µm | 0.4 µm |
| Flatness (over 100 mm) | 0.05 mm | 0.013 mm | 0.005 mm |
| Parallelism (over 100 mm) | 0.05 mm | 0.013 mm | 0.005 mm |
| Concentricity / true position | 0.05 mm | 0.025 mm | 0.013 mm |
| Angle accuracy | ±0.5° | ±0.1° | ±0.05° |
Tolerance stack-up — the silent cost driver
When several toleranced features have to interact, their individual tolerances add up. This is “tolerance stack-up” — and ignoring it leads to assemblies that fail QA even though every individual part is within spec.
Worked example: a stack of three identical washers, each toleranced ±0.05 mm thick:
Per-washer tolerance
Each washer can be 0.05 mm too thin or 0.05 mm too thick. Range = 0.10 mm per washer.
Worst-case stack
Three washers at the worst end: 3 × 0.05 = 0.15 mm thinner OR 0.15 mm thicker than nominal. Total range: 0.30 mm.
Statistical stack (RSS)
In practice not every washer is at the extreme. Root-Sum-Square gives more realistic ±0.087 mm at 3-sigma.
Design implication
If your assembly needs to fit in a 25 ±0.10 mm slot, 75 ±0.30 mm worst-case won’t fit. You must either tighten individual tolerances OR widen the slot OR redesign.
GD&T (Geometric Dimensioning and Tolerancing) in 5 minutes
GD&T is a symbolic language (ASME Y14.5 / ISO 1101) for specifying not just dimensions but the geometric relationships between features. It’s how aerospace, automotive and medical drawings communicate what really matters.
| Symbol | Name | Controls |
|---|---|---|
| — | Straightness | How straight a line/axis is |
| ◯ (circle) | Circularity / roundness | How round a cross-section is |
| ⌭ | Cylindricity | 3D roundness over the length of a cylinder |
| ▱ (parallelogram) | Flatness | How flat a surface is |
| ∥ | Parallelism | Surface parallel to a datum |
| ⊥ | Perpendicularity | Surface perpendicular to a datum |
| ∠ | Angularity | Surface at a specific angle to a datum |
| ⌖ | True position | Where a feature is, relative to datums |
| ◎ | Concentricity | Whether two cylinders share an axis |
| ⌭ R | Runout | Combined error during rotation |
| Σ | Profile | Allowable variation of a curved surface |
Two key concepts make GD&T more powerful than “plus-minus” tolerancing:
Datums
- A datum (A, B, C…) is a feature you reference everything else from.
- Establishes a coordinate system on the part.
- Without datums, “perpendicular” is ambiguous — perpendicular to what?
Bonus tolerance
- GD&T allows extra tolerance when a feature is at maximum material condition (MMC).
- A hole at its smallest allowed size has more “bonus” positional tolerance.
- Lets the shop produce in-spec parts that “plus-minus” alone would reject.
How to specify tolerances on a drawing
Use a title-block default
Top right of the drawing: “General tolerance: ISO 2768-mK” (or ASME equivalent). Now most dimensions don’t need explicit tolerances.
Tolerance only what matters
Mating dimensions, sealing faces, bearing seats, datums. Leave decorative or non-functional features at the default.
Use the tightest tolerance only where required
A typical optimised drawing has 3–5 features at ±0.025 mm and the rest at ±0.1 mm default.
Specify surface finish where it matters
Use the standard finish symbol (✓ with Ra value) on the surfaces that need it. “Ra 0.8” on a sealing face; rest defaults.
Add datums for GD&T-controlled features
Pick the most-stable, most-machined surface as datum A. Usually a large flat face. Datum B and C are perpendicular to A.
How tolerances are verified
| Tool | Best for | Tolerance reach |
|---|---|---|
| Steel rule | Rough check during machining | ±0.5 mm |
| Vernier / digital caliper | General-purpose checking | ±0.05 mm |
| Micrometer | External diameters, thicknesses | ±0.005 mm |
| Bore gauge | Internal diameters | ±0.005 mm |
| Pin gauge / plug gauge | Hole sizes (go / no-go) | IT class fit |
| Height gauge with indicator | Heights, perpendicularity | ±0.01 mm |
| Surface plate + indicator | Flatness, parallelism over 100s of mm | ±0.005 mm |
| CMM (Coordinate Measuring Machine) | Complex features, GD&T verification, FAI | ±0.002 mm |
| Optical comparator | Profiles, threads, sharp corners | ±0.005 mm |
| Surface roughness tester | Ra, Rz measurements | 0.01 µm |
Cost impact of tightening tolerances
Indicative cost multiplier vs the same feature at default ±0.10 mm:
| Tolerance | Cost multiplier | Why |
|---|---|---|
| ±0.10 mm (default) | 1.0× | Standard cycle, hand-held inspection. |
| ±0.05 mm | 1.2× | Slightly slower cuts, may need micrometer. |
| ±0.025 mm | 1.5× | Quality CNC machine, micrometer or bore gauge inspection. |
| ±0.013 mm | 2× | Modern 5-axis or grinding machine, CMM verification. |
| ±0.005 mm | 3–5× | Top-tier machine, climate-controlled cell, full CMM, sometimes hand finishing. |
| ±0.002 mm | 10×+ | Hand lapping, jig grinding, hours of inspection per part. |
Common mistakes to avoid
- Tightening every dimension to ±0.005 mm. The classic rookie mistake. Triples cost for no functional benefit.
- No general-tolerance call-out. Without ISO 2768 in the title block, every dimension becomes ambiguous and the shop will quote conservatively.
- Toleranced angles smaller than ±0.5°. Most CNC mills handle ±0.5° easily. Tighter angles often require fixturing or grinding.
- Demanding mirror surface (Ra 0.05) on functional surfaces. Polishing adds significant cost and lead time. Use Ra 0.8 unless optical or sealing function actually requires better.
- Overlapping datum schemes. If A is the bottom and B is the side, don’t also reference the bottom as B somewhere else. Pick a clean A-B-C scheme and stick with it.
- Forgetting MMC modifiers. True position with no modifier is the strictest interpretation. Adding Ⓜ (MMC) gives the shop legitimate bonus tolerance — use it where appropriate.
- Tolerancing what can’t be measured. If the only inspection tool that can verify the spec costs $250k, expect to pay for that inspection.
Foire aux questions
- 0,005 mm représente environ 1/10 de l'épaisseur d'un cheveu humain. Vous ne pouvez ni le voir, ni le sentir, ni le mesurer avec un pied à coulisse. Cela nécessite au minimum un micromètre calibré, idéalement une MMT.
- Le tolérancement plus-minus suffit pour les pièces grand public non-assemblées. Utilisez le GD&T quand la fonction dépend des relations entre éléments ou si le client est dans une industrie réglementée.
- ISO 2768 est la norme internationale pour les "tolérances générales" — valeurs par défaut pour toute dimension non explicitement tolérancée. La classe "m" (moyenne) est la plus courante.
- Oui, sur le bon matériau et géométrie. Nous avons plusieurs cellules 5 axes et une MMT à température contrôlée. Pour l'aérospatial et le médical, nous atteignons régulièrement ±0,005 mm avec vérification à 100%.
- Les outils manuels sont rapides et bon marché pour les dimensions individuelles. La MMT est requise pour la position réelle, les relations multi-éléments, les surfaces libres et la documentation FAI.
- Inspection documentée de la toute première pièce produite, vérifiant chaque dimension contre le modèle CAO. La FAI conforme à AS9102 est requise pour les commandes aérospatiales.
- Pour des éléments spécifiques oui — sièges de roulement rectifiés, surfaces d'étanchéité rodées, alésages à la pointe atteignent ±0,002 mm. Ce sont des opérations spécialisées avec coûts et délais importants.
À quoi ressemble vraiment ±0,005 mm ?
Dois-je toujours spécifier le GD&T ?
Qu'est-ce qu'ISO 2768 ?
JLYPT peut-il atteindre ±0,005 mm de façon constante ?
Comment choisir entre micromètre et MMT ?
Qu'est-ce qu'une First Article Inspection (FAI) ?
Puis-je obtenir des tolérances plus serrées que ±0,005 mm ?
À propos de l'auteur
JLYPT Engineering Team
Senior CNC Application Engineers
Our application engineering team brings 15+ years of combined experience producing precision components for aerospace, medical, robotics and industrial automation customers.
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